This invention relates to wet flue gas desulfurizers. More particularly, it relates to a wet flue gas desulfurizer for removing SO2 from the exhaust gas of a boiler or the like by wet desulfurization, and an oxygen-containing gas blowing device for use therein.
When a sulfur-containing fuel is burned, the sulfur is discharged into the atmosphere in the form of sulfur dioxide (SO2), except for that fixed in ash. This sulfur dioxide exerts a significant harmful influence not only on human beings and animals, but also on the environment by causing acid rain to fall on the earth.
For this reason, large-scale combustion facilities and plants have conventionally been equipped with flue gas desulfurizers, most of which are wet flue gas desulfurizers.
In such a wet desulfurization process, exhaust gas is brought into gas-liquid contact with an absorbing fluid containing an alkali such as lime, so that SO2 is absorbed and removed from the exhaust gas. As a result, the SO2 absorbed from the exhaust gas forms sulfites in the absorbing fluid. In order to oxidize these sulfites and thereby form stable sulfates, it has been common practice to oxidize them by blowing air into the absorbing fluid.
Commonly employed wet flue gas desulfurizers are of the so-called tank oxidation type. In this type of wet flue gas desulfurizer, an oxygen-containing gas (typically air) is blown into an tank of an absorption tower, where it is brought into contact with a slurry (containing a calcium compound such as limestone) having sulfur dioxide absorbed therein so as to oxidize the sulfur dioxide. Thus, the necessity of an oxidation tower is eliminated.
In this case, it is necessary to bring air into efficient contact with the slurry in the aforesaid tank. For this purpose, various methods and devices have been known in the prior art.
FIG. 7 illustrates an oxygen feeding device described in Japanese Patent Provisional Publication No. 61-74630. In this device, air is blown into a slurry oxidation tank 201 by means of an oxygen feeding means 202. The blown-in air is dispersed with an agitator 203 to improve the efficiency of air utilization.
However, the conventional device of FIG. 7 has difficulties in that unduly great power is required to agitate the slurry within oxidation tank 201, and the efficiency of utilization of the injected oxygen is low.
Moreover, FIG. 8 illustrates a device constructed according to Japanese Patent Publication No. 6-91939. Also in this device, air is blown into a region in the vicinity of an agitator 302 by means of an oxygen feeding means 303. However, the device of FIG. 8 also has difficulties in that unduly great power is required to agitate the slurry within oxidation tank 301, and the efficiency of utilization of the injected oxygen is low.
The present inventor has also employed a stationary oxidation device in prior art oxidation equipment (Japanese Patent Provisional Publication No. 9-10546). However, owing to its low efficiency of oxygen utilization (usually about 20%), the flow rate of oxidizing air or the number of sprayer nozzles must be increased to cause problems from the viewpoint of equipment cost and maintainability.
Moreover, in the device which has conventionally been employed by the present inventor as described in Japanese Patent Provisional Publication No. 9-10546 or the like, the sprayer itself is rotated to produce finely divided air bubbles and agitate the liquid at the same time. However, since this device masticates air, a loss in agitation capability is caused and unduly great power is required.
On the other hand, FIG. 9 illustrates a wet flue gas desulfurizer having such an oxygen-containing gas blowing means (hereinafter referred to as a first example of the prior art). As shown in FIG. 9, this wet flue gas desulfurizer is constructed so that it has an absorption tower 2 for effecting wet desulfurization, a fluid reservoir 31 for storing an absorbing fluid b containing an alkaline absorbent d (e.g., lime) is disposed below absorption tower 2, and the absorbing fluid b within fluid reservoir 31 is raised by means of a circulating pump 6 and sprayed through spray pipes 30 disposed in the lower part of absorption tower 2.
In the above-described equipment construction, combustion exhaust gas a is introduced into absorption tower 2 at the top thereof, and brought into gas-liquid contact with the alkali-containing absorbing fluid sprayed through spray pipes 30 so as to absorb and remove SO2 from the exhaust gas. Thereafter, the cleaned exhaust gas c is discharged to the outside through an exhaust duct 38. The absorbing fluid in which sulfites have been formed from SO2 absorbed from the exhaust gas is returned to fluid reservoir 31, where the aforesaid sulfites are oxidized to sulfates with the aid of an oxygen-containing gas e fed by an oxygen-containing gas blowing means. Thereafter, an amount of the sulfates which is stoichiometrically equivalent to that of SO2 absorbed are discharged as waste fluid f by way of circulating pump 6 and a pipeline 40. On the other hand, the alkaline absorbent d (e.g., lime) is supplied to the aforesaid fluid reservoir 31 by way of a pipeline 39.
The aforesaid oxygen-containing gas blowing means comprises a plurality of nozzle headers 102 which extend from the sidewall of fluid reservoir 31 in a downwardly inclined direction and are each equipped with a plurality of feed nozzles 101. These nozzle headers 102 are disposed so as to cover substantially the entire bottom surface of fluid reservoir 31. Thus, using a blower 36, oxygen-containing gas e is fed through a pipeline 37 and nozzle headers 102, and injected from the injection orifices 35 of feed nozzles 101 into the absorbing fluid b for the purpose of oxidizing the sulfites.
FIG. 10 illustrates the construction of a wet flue gas desulfurizer having another oxygen-containing gas blowing means. This represents a practically employed oxidation method in which an injection orifice 35 at the tip of a pipeline 37 is disposed in front of an agitator 203 attached to the sidewall of fluid reservoir 31, and an oxygen-containing gas e fed through pipeline 37 by means of a blower 36 is injected from injection orifice 35 into the absorbing fluid b so as to promote the dispersion of oxygen-containing gas e by the jet of the absorbing fluid b that is driven by the aforesaid agitator 203. (This oxygen-containing gas blowing means will hereinafter be referred to as a second example of the prior art.)
Moreover, as illustrated in FIG. 11, Japanese Utility Model Provisional Publication No. 4-137731 discloses an oxygen-containing gas blowing means comprising a plurality of jet nozzles 151 for injecting a jet of an absorbing fluid in a direction forming a predetermined angle with the corresponding diameter of a fluid reservoir 150. These jet nozzles 151 are attached to the sidewall of fluid reservoir 150 at a predetermined vertical position so as to inject the absorbing fluid in the circumferential direction of the sidewall. The basal end of each jet nozzle 151 is provided with an absorbing fluid pipeline 153 communicating with fluid reservoir 150 and having a jet pump 152 installed in an intermediate part thereof, and a gas pipe 154 is connected to absorbing fluid pipeline 153 between jet pump 152 and jet nozzle 151. (This oxygen-containing gas blowing means will hereinafter be referred to as a third example of the prior art.)
Furthermore, a further oxygen-containing gas blowing means has been known. Specifically, as illustrated in FIG. 12, a delivery pipe 161 is attached so as to penetrate into a fluid reservoir 160 through the sidewall thereof, and connected with a circulating fluid pipe 163 for sucking out an absorbing fluid from fluid reservoir 160 and circulating it by means of a fluid pump 162. Moreover, a gas blowing pipe 164 is attached so as to penetrate into an intermediate part of the aforesaid circulating fluid pipe 163, and its outlet part 164a is bent at the center of circulating fluid pipe 163 so as to be open in the direction of fluid flow. Thus, an oxygen-containing gas is fed through gas blowing pipe 164 under pressure by means of a blower 165, blown into circulating fluid pipe 163, and discharged from delivery pipe 161 together with the absorbing fluid. (This oxygen-containing gas blowing means will hereinafter be referred to as a fourth example of the prior art.)
Although all of the above-described oxygen-containing gas blowing means in accordance with the first to fourth examples of the prior art provide excellent oxidation methods, they involve the following problems.
For example, the first example of the prior art illustrated in FIG. 9 has the disadvantage that, since a large number of feed nozzles 101 are disposed so as to cover substantially the whole bottom surface of fluid reservoir 31, this may hinder inspection and other operations in fluid reservoir 31.
In the second example of the prior art illustrated in FIG. 10, as a result of an upward flow induced by the air-lifting action of oxygen-containing gas e injected from injection orifice 35, agitating blades 204 tend to cause a local circulation in which a portion of the fluid just delivered is sucked in again, resulting in reduced agitation efficiency. Moreover, this also decreases the throw of the discharged fluid, resulting in reduced agitation capability. Accordingly, it is necessary to maintain the agitation capability by strengthening the agitation power.
In the third and fourth examples of the prior art illustrated in FIGS. 11 and 12, a gas is fed to an intermediate part of absorbing fluid pipeline 153 or 163 connected to jet nozzle 151 or delivery pipe 161. Consequently, while gas bubbles flow through the absorbing fluid pipeline together with the absorbing fluid, some of them may combine together to form coarse bubbles, or the gas and the absorbing fluid may separate into discrete phases. When the gas is discharged from jet nozzle 151 or delivery pipe 161 in such a state, it cannot be uniformly dispersed and fails to achieve smooth oxidation. Moreover, the gas tends to cause the problem of erosion of the internal surface of the absorbing fluid pipe due to cavitation.
In view of this existing state of the art, an object of the present invention is to provide a wet flue gas desulfurizer which does not require unduly great power in order to agitate the slurry within the slurry oxidation tank, and can enhance the efficiency of utilization of the injected oxygen.
Moreover, the present invention has been made with a view to overcoming the above-described disadvantages. Accordingly, another object of the present invention is to provide an oxygen-containing gas blowing means for use in a wet flue gas desulfurizer which can achieve a marked reduction in the number of feed nozzles, a reduction in power, and a marked improvement in the capacity for agitating and dispersing the absorbing fluid.
In order to accomplish the above objects, the present invention provides a wet flue gas desulfurizer wherein sulfur dioxide is absorbed into a slurry and an oxygen-containing gas is blown into the slurry within a slurry oxidation tank to oxidize sulfites present in the slurry, characterized in that the slurry oxidation tank is equipped with a return pipeline for returning a portion of the slurry to a position at or near the bottom of the slurry oxidation tank, and the oxygen-containing gas is blown in at the discharge end of the return pipeline so as to divide the oxygen-containing gas finely by the action of the slurry returned through the return pipeline.
It is a preferred embodiment that, in the above-described wet flue gas desulfurizer, a portion of the slurry stored in the slurry oxidation tank is withdrawn and returned through the return pipeline.
It is another preferred embodiment that, in the above-described wet flue gas desulfurizer, the slurry injected from header pipes is returned through the return pipeline.
It is still another preferred embodiment that, in the above-described wet flue gas desulfurizer, the slurry collected by a mist eliminator is returned through the return pipeline.
It is a further preferred embodiment that, in the above-described wet flue gas desulfurizer, when a portion of the slurry is withdrawn at a position near the bottom of the slurry oxidation tank and sent to header pipes under pressure by means of a pressure pump, a slurry delivery pipe is branched from the pipeline for sending the slurry to the header pipes, and the oxygen-containing gas is blown in at the discharge end of the delivery pipe so as to divide the oxygen-containing gas finely by the action of the slurry discharged from the delivery pipe.
The present invention also provides an oxygen-containing gas blowing device for use in a wet flue gas desulfurizer for removing SO2 from combustion exhaust gas by wet desulfurization, wherein a fluid reservoir for an absorbing fluid is equipped with a delivery pipe for discharging the absorbing fluid so that its discharge end is open in the fluid reservoir, and an oxygen feed nozzle for injecting an oxygen-containing gas is disposed in the area of the discharged stream in the neighborhood of the discharge end of the delivery pipe (i.e., just behind or before the discharge end).
According to the present invention, the jet of the absorbing fluid discharged from the discharge end at the tip of the absorbing fluid delivery pipe is accompanied by a wake and hence moves in the direction of discharge at an increased flow rate. Thus, the oxygen-containing gas injected from the oxygen feed nozzle can be widely dispersed in the form of gas bubbles while overcoming an upward flow induced by its air-lifting action.
Consequently, the number of oxygen feed nozzles can be markedly reduced. Moreover, since the delivery part may be disposed apart from the suction part for sucking in the absorbing fluid by means of a pump, no local circulation is caused. Furthermore, since the discharged jet of the absorbing fluid is accompanied by a wake and hence moves in the direction of discharge at an increased flow rate, high efficiency can be achieved in the dispersion of gas bubbles without requiring any extra power.
Moreover, according to the present invention, an oxygen-containing gas is injected into the area of the discharged stream in the neighborhood of the discharge end of the delivery pipe (i.e., just behind or before the discharge end), in contrast to the third and fourth examples of the prior art in which an oxygen-containing gas is fed to a fluid pipeline on the upstream side of a delivery pipe. Thus, only the absorbing fluid is supplied up to the discharge end, so that the above-described problems, such as the combination of gas bubbles in the fluid pipeline, the separation of the gas from the absorbing fluid, and the resulting poor dispersion of discharged gas bubbles and poor performance of the fluid pipeline, can be solved. Moreover, since the injected gas collides with the discharged stream at a position just behind or before the discharge end where the discharged stream has the highest flow velocity, the injected gas is finely divided and smoothly dispersed to achieve the smooth oxidation of sulfites over a wide area.
In the device of the present invention, an injection orifice at the tip of the oxygen feed nozzle may effectively be disposed in the area of the jet discharged from the aforesaid delivery pipe. Thus, the oxygen-containing gas injected from the oxygen feed nozzle can be dispersed in the form of fine gas bubbles.
Moreover, the device of the present invention may be constructed in such a way that the upper part of the tip of the delivery pipe is made longer so as to overhang the lower part thereof, and the oxygen feed nozzle is attached so as to extend through the overhanging part. Thus, the oxygen-containing gas injected from the oxygen feed nozzle can be prevented from blowing through.
Furthermore, the aforesaid delivery pipe may be inclined downward in the fluid reservoir. This makes it possible to prevent solid matter from depositing in the delivery tube or flowing back thereinto when the circulation of the absorbing fluid by means of a pump is stopped. In this case, the delivery pipe may be inclined until it stands upright.
Furthermore, the device of the present invention may be constructed in such a way that the aforesaid oxygen feed nozzle penetrates into the delivery pipe at a position before the discharge end of the delivery pipe. Thus, the oxygen-containing gas can be dispersed in the form of finer gas bubbles. Moreover, since the residence time of a gas-liquid mixed flow in the delivery pipe is minimized, it is possible to minimize the damage of the delivery pipe by a gas-liquid mixed flow while overcoming the disadvantages of the above-described third and fourth examples of the prior art.
Furthermore, the device of the present invention may be constructed in such a way that the aforesaid delivery pipe penetrates into the fluid reservoir through the sidewall thereof, and the delivery pipe is radially deflected toward a tangential direction so as to cause the stream discharged from the delivery pipe to flow along the sidewall of the fluid reservoir. Thus, a circular flow can be produced in the aforesaid fluid reservoir so as to further prolong the gas-liquid contact time between the oxygen-containing gas and the absorbing fluid.